The ABS function is fully automatic. All you need to do is supply a number and ABS will return the absolute value.
Convert negative numbers in place
If you only need to convert negative numbers once, you can convert in-place with Paste Special:
- Add -1 to a cell and copy to the clipboard
- Select the negative numbers you want to convert
- Use Paste Special > Values + Multiply
The video on this page shows this technique and many other paste special shortcuts.